Home baked goods are the best, but they generally don't travel well or last long.
Varieties of packaged snacks. Something to break up the monotony of items 1-20 you eat on a regular basis. Quality isn't as big of a deal as variety...for example when dudes from Japan send a box full of 100 yen store junk food, it's a pretty big hit because of all the weird shit they have. Also it helps to have a few things that have a longer shelf life to throw in the helmet bag for missed meals.
One time someone sent a huge box of unshelled peanuts. It was a pretty big hit and the ops floor forever looked like it was a texas roadhouse. You could go to a bulk grocery store and fill up a box with mixes and misc things like that.
Sauces or salsa are good too. You can spice up a lot of shitty DFAC food with some sriracha or BW3 sauces. Gum. Mints. Water flavoring packets, if they have caffeine it's a bonus. Think crystal light, or something like the true lime packets.
Air freshener things like candles or something you can ship. Everyone stinks, they just don't know it until a female walks within 100m and you regain your superhuman olfactory senses.
One time someone sent a box full of silly string canisters and 30 minutes later all the string was gone and we had to break up a fist fight. Board games would generally do well...check your local thrift store or garage sales for cheap ones.
Holiday items work as well. Santa hats and beards, discount halloween costume items...there's always a squadron clown that will put them to good use at the most inappropriate time. Rubber snakes, spiders, etc. would keep things lively for a bit. After-season discount christmas lights.
Coffee is always a hit, and so are a variety of teas. Tylenol/Advil PM is the poor man's ambien when you've got a stingy flight doc.
Also Amazon has kind of revolutionized deployments with free shipping. If you have a unit's direct address you could easily just amazon random stuff to them cheaper than any USPS box. Keeping an eye out on slickdeals.net you will always see wipes, food, coffee, etc. that can be sent via Prime for a fraction of the price you could do it yourself. If it can be eaten, it will.
Also there is space-a mail for cheaper rates if whatever you are shipping can last a few months in transit.